deb-python-wrapt/docs/decorators.rst

Decorators

The wrapt module provides various components, but the main reason that it likely is to be used is for creating decorators. This document covers the creation of decorators and all the information needed to cover what you can do within the wrapper function linked to your decorator.

Creating Decorators

To implement your decorator you need to first define a wrapper function. This will be called each time a decorated function is called. The wrapper function needs to take four positional arguments:

• wrapped - The wrapped function which in turns needs to be called by your wrapper function.
• instance - The object to which the wrapped function was bound when it was called.
• args - The list of positional arguments supplied when the decorated function was called.
• kwargs - The dictionary of keyword arguments supplied when the decorated function was called.

The wrapper function would do whatever it needs to, but would usually in turn call the wrapped function that is passed in via the wrapped argument.

The decorator @wrapt.decorator then needs to be applied to the wrapper function to convert it into a decorator which can in turn be applied to other functions.

import wrapt

@wrapt.decorator
def pass_through(wrapped, instance, args, kwargs):
    return wrapped(*args, **kwargs)

@pass_through
def function():
    pass

Decorators With Arguments

If you wish to implement a decorator which accepts arguments, then wrap the definition of the decorator in a function closure. Any arguments supplied to the outer function when the decorator is applied, will be available to the inner wrapper when the wrapped function is called.

import wrapt

def with_arguments(myarg1, myarg2):
    @wrapt.decorator
    def wrapper(wrapped, instance, args, kwargs):
        return wrapped(*args, **kwargs)
    return wrapper

@with_arguments(1, 2)
def function():
    pass

If using Python 3, you can use the keyword arguments only syntax to force use of keyword arguments when the decorator is used.

import wrapt

def with_keyword_only_arguments(*, myarg1, myarg2):
    @wrapt.decorator
    def wrapper(wrapped, instance, args, kwargs):
        return wrapped(*args, **kwargs)
    return wrapper

@with_keyword_only_arguments(myarg1=1, myarg2=2)
def function():
    pass

Decorators With Optional Arguments

Although opinion can be mixed about whether the pattern is a good one, if the decorator arguments all have default values, it is also possible to implement decorators which have optional arguments. This allows the decorator to be applied with or without the arguments, with the brackets being able to be dropped in the latter.

import wrapt

def with_optional_arguments(wrapped=None, myarg1=1, myarg2=2):
    if wrapped is None:
        return functools.partial(with_optional_arguments,
                myarg1=myarg1, myarg2=myarg2)

    @wrapt.decorator
    def wrapper(wrapped, instance, args, kwargs):
        return wrapped(*args, **kwargs)

    return wrapper(wrapped)

@with_optional_arguments(myarg1=1, myarg2=2)
def function():
    pass

@with_optional_arguments
def function():
    pass

For this to be used in this way, it is a requirement that the decorator arguments be supplied as keyword arguments.

If using Python 3, the requirement to use keyword only arguments can again be enforced using the keyword only argument syntax.

import wrapt

def with_optional_arguments(wrapped=None, *, myarg1=1, myarg2=2):
    if wrapped is None:
        return functools.partial(with_optional_arguments,
                myarg1=myarg1, myarg2=myarg2)

    @wrapt.decorator
    def wrapper(wrapped, instance, args, kwargs):
        return wrapped(*args, **kwargs)

    return wrapper(wrapped)

Processing Function Arguments

The original set of positional arguments and keyword arguments supplied when the decorated function is called will be passed in the args and kwargs arguments.

Note that these are always passed as their own unique arguments and are not broken out and bound in any way to the decorator wrapper arguments. In other words, the decorater wrapper function signature must always be:

@wrapt.decorator
def my_decorator(wrapped, instance, args, kwargs): # CORRECT
    return wrapped(*args, **kwargs)

You cannot use:

@wrapt.decorator
def my_decorator(wrapped, instance, *args, **kwargs): # WRONG
    return wrapped(*args, **kwargs)

nor can you specify actual named arguments to which args and kwargs would be bound.

@wrapt.decorator
def my_decorator(wrapped, instance, arg1, arg2): # WRONG
    return wrapped(arg1, arg2)

Separate arguments are used and no binding performed to avoid the possibility of name collisions between the arguments passed to a decorated function when called, and the names used for the wrapped and instance arguments. This can happen for example were wrapped and instance also used as keyword arguments by the wrapped function.

If needing to modify certain arguments being supplied to the decorated function when called, you will thus need to trigger binding of the arguments yourself. This can be done using a nested function which in turn then calls the wrapped function:

@wrapt.decorator
def my_decorator(wrapped, instance, args, kwargs):
    def _execute(arg1, arg2, *_args, **_kwargs):

        # Do something with arg1 and arg2 and then pass the
        # modified values to the wrapped function. Use 'args'
        # and 'kwargs' on the nested function to mop up any
        # unexpected or non required arguments so they can
        # still be passed through to the wrapped function.

        return wrapped(arg1, arg2, *_args, **_kwargs)

    return _execute(*args, **kwargs)

If you do not need to modify the arguments being passed through to the wrapped function, but still need to extract them so as to log them or otherwise use them as input into some process you could instead use.

@wrapt.decorator
def my_decorator(wrapped, instance, args, kwargs):
    def _arguments(arg1, arg2, *args, **kwargs):
        return (arg1, arg2)

    arg1, arg2 = _arguments(*args, **kwargs)

    # Do something with arg1 and arg2 but still pass through
    # the original arguments to the wrapped function.

    return wrapped(*args, **kwargs)

You should not simply attempt to extract positional arguments from args directly because this will fail if those positional arguments were actually passed as keyword arguments, and so were passed in kwargs with args being an empty tuple.

Function Argument Specifications

To obtain the argument specification of a decorated function the standard getargspec() function from the inspect module can be used.

@wrapt.decorator
def my_decorator(wrapped, instance, args, kwargs):
    return wrapped(*args, **kwargs)

@my_decorator
def function(arg1, arg2):
    pass

>>> print(inspect.getargspec(function))
ArgSpec(args=['arg1', 'arg2'], varargs=None, keywords=None, defaults=None)

If using Python 3, the getfullargspec() or signature() functions from the inspect module can also be used.

In other words, applying a decorator created using @wrapt.decorator to a function is signature preserving and does not result in the loss of the original argument specification as would occur when more simplistic decorator patterns are used.

Wrapped Function Documentation

To obtain documentation for a decorated function which may be specified in a documentation string of the original wrapped function, the standard Python help system can be used.

@wrapt.decorator
def my_decorator(wrapped, instance, args, kwargs):
    return wrapped(*args, **kwargs)

@my_decorator
def function(arg1, arg2):
    """Function documentation."""
    pass

>>> help(function)
Help on function function in module __main__:

function(arg1, arg2)
    Function documentation.

Just the documentation string itself can still be obtained by accessing the __doc__ attribute of the decorated function.

>>> print(function.__doc__)
Function documentation.

Wrapped Function Source Code

To obtain the source code of a decorated function the standard getsource() function from the inspect module can be used.

@wrapt.decorator
def my_decorator(wrapped, instance, args, kwargs):
    return wrapped(*args, **kwargs)

@my_decorator
def function(arg1, arg2):
    pass

>>> print(inspect.getsource(function))
@my_decorator
def function(arg1, arg2):
    pass

As with signatures, the use of the decorator does not prevent access to the original source code for the wrapped function.

Signature Changing Decorators

When using inspect.getargspec() the argument specification for the original wrapped function is returned. If however the decorator is a signature changing decorator, this is not going to be what is desired.

In this circumstance it is necessary to pass a dummy function to the decorator via the optional adapter argument. When this is done, the argument specification will be sourced from the prototype for this dummy function.

def _my_adpater_prototype(arg1, arg2): pass

@wrapt.decorator(adapter=_my_adpater_prototype)
def my_adapter(wrapped, instance, args, kwargs):
    """Adapter documentation."""

    def _execute(arg1, arg2, *_args, **_kwargs):

        # We actually multiply the first two arguments together
        # and pass that in as a single argument. The prototype
        # exposed by the decorator is thus different to that of
        # the wrapped function.

        return wrapped(arg1*arg2, *_args, **_kwargs)

    return _execute(*args, **kwargs)

@my_adapter
def function(arg):
    """Function documentation."""

    pass

>>> help(function)
Help on function function in module __main__:

function(arg1, arg2)
    Function documentation.

As it would not be accidental that you applied such a signature changing decorator to a function, it would normally be the case that such usage would be explained within the documentation for the wrapped function. As such, the documentation for the wrapped function is still what is used for the __doc__ string and what would appear when using the Python help system. In the latter, the arguments required of the adapter would though instead appear.

Decorating Functions

When applying a decorator to a normal function, the instance argument would always be None.

@wrapt.decorator
def pass_through(wrapped, instance, args, kwargs):
    return wrapped(*args, **kwargs)

@pass_through
def function(arg1, arg2):
    pass

function(1, 2)

Decorating Instance Methods

When applying a decorator to an instance method, the instance argument will be the instance of the class on which the instance method is called. That is, it would be the same as self passed as the first argument to the actual instance method.

@wrapt.decorator
def pass_through(wrapped, instance, args, kwargs):
    return wrapped(*args, **kwargs)

class Class(object):

    @pass_through
    def function_im(self, arg1, arg2):
        pass

c = Class()

c.function_im(1, 2)

Class.function_im(c, 1, 2)

Note that the self argument is only passed via instance, it is not passed as part of args. Only the arguments following on from the self argument will be a part of args.

When calling the wrapped function in the decorator wrapper function, the instance should never be passed explicitly though. This is because the instance is already bound to wrapped and will be passed automatically as the first argument to the original wrapped function.

This is even the situation where the instance method was called via the class type and the self pointer passed explicitly. This is the case as the decorator identifies this specific case and adjusts instance and args so that the decorator wrapper function does not see it as being any different to where it was called directly on the instance.

Decorating Class Methods

When applying a decorator to a class method, the instance argument will be the class type on which the class method is called. That is, it would be the same as cls passed as the first argument to the actual class method.

@wrapt.decorator
def pass_through(wrapped, instance, args, kwargs):
    return wrapped(*args, **kwargs)

class Class(object):

    @pass_through
    @classmethod
    def function_cm(cls, arg1, arg2):
        pass

Class.function_cm(1, 2)

Note that the cls argument is only passed via instance, it is not passed as part of args. Only the arguments following on from the cls argument will be a part of args.

When calling the wrapped function in the decorator wrapper function, the instance should never be passed explicitly though. This is because the instance is already bound to wrapped and will be passed automatically as the first argument to the original wrapped function.

Note that due to a bug in Python classmethod.__get__(), whereby it does not apply the descriptor protocol to the function wrapped by @classmethod, the above only applies where the decorator wraps the @classmethod decorator. If the decorator is placed inside of the @classmethod decorator, then instance will be None and the decorator wrapper function will see the call as being the same as a normal function. As a result, always place any decorator outside of the @classmethod decorator. Hopefully this issue in Python can be addressed in Python 3.4.

Decorating Static Methods

When applying a decorator to a static method, the instance argument will be None. In other words, the decorator wrapper function will not be able to distinguish a call to a static method from a normal function.

@wrapt.decorator
def pass_through(wrapped, instance, args, kwargs):
    return wrapped(*args, **kwargs)

class Class(object):

    @pass_through
    @staticmethod
    def function_sm(arg1, arg2):
        pass

Class.function_sm(1, 2)

Decorating Classes

When applying a decorator to a class, the instance argument will be None. In order to distinguish this case from a normal function call, inspect.isclass() should be used on wrapped to determine if it is a class type.

@wrapt.decorator
def pass_through(wrapped, instance, args, kwargs):
    return wrapped(*args, **kwargs)

@pass_through
class Class(object):
    pass

c = Class()

Universal Decorators

A universal decorator is one that can be applied to different types of functions and can adjust automatically based on what is being decorated.

For example, the decorator may be able to be used on both a normal function and an instance method, thereby avoiding the need to create two separate decorators to be used in each case.

A universal decorator can be created by observing what has been stated above in relation to the expected values/types for wrapped and instance passed to the decorator wrapper function.

These rules can be summarised by the following.

import inspect

@wrapt.decorator
def universal(wrapped, instance, args, kwargs):
    if instance is None:
        if inspect.isclass(wrapped):
            # Decorator was applied to a class.
            return wrapped(*args, **kwargs)
        else:
            # Decorator was applied to a function or staticmethod.
            return wrapped(*args, **kwargs)
    else:
        if inspect.isclass(instance):
            # Decorator was applied to a classmethod.
            return wrapped(*args, **kwargs)
        else:
            # Decorator was applied to an instancemethod.
            return wrapped(*args, **kwargs)

To be truly robust, a universal decorator should raise a runtime exception at the point it is subsequently called, when it was applied as a decorator in a scenario it does not support.